1. Field of the Invention
This invention relates to the field of power assist apparatus to be attached to human beings and the like, for assisting the application of muscle power to move skeletal joints. In particular the invention provides power assist apparatus that is mechanically coupled to the wearer in a manner that aligns the application of force by the apparatus to the rotational axes of the wearer's joints.
2. Prior Art
Exoskeletal power assist devices are known, for example, from U.S. Pat. No. 1,880,138 Hubl; U.S. Pat. No. 3,358,678 Kultsar; U.S. Pat. No. 3,449,769--Mizen; U.S. Pat. No. 4,180,870--Radulovic; and U.S. Pat. No. 4,557,257--Fernandez et al. These disclosures range from Hubl, wherein the device is much larger than the operator or "wearer", to Kultsar, wherein the device is embodied as a form-fitting suit including power applying apparatus. The general object may be to sense the application of force from the wearer's muscle power, and to operate a power assist mechanism that applies force in the same direction, i.e., to multiply the force applied by the wearer. Alternatively, the device may provide controlled joint displacement for a wearer whose muscles or nerves are inoperative for applying force in a desired direction.
Power assist devices of this kind may rely on the wearer's skeletal joints for constraining movement of the skeletal members at a joint, namely with power assisting actuators being fixed relative to the wearer's skeletal members The opposite ends of an extendible/retractable actuator are attached respectively to the wearer on opposite sides of the joint. The means to which the ends of the actuator are attached may define at least a partial rigid supplemental skeletal member which is attached along the user's limbs or the like on opposite sides of one or more joints. An actuator coupled to a controller applies power to the supplemental skeletal member(s) to cause relative rotation of the wearer's skeletal members about the joint. For a more heavy duty application, the device may have a more complete version of the skeletal members, in the form of rigid structures which are placed alongside the user's skeletal members and are attached to one another at joints which supplement the wearer's joints. For a knee supplement, for example, a first rigid member is attached along the femur and a second rigid member is attached along the tibia and fibula, the two rigid members being affixed to one another via a hinging joint. Similarly, and elbow supplement would have an external humerus attachment hinged to a radius/ulna attachment. Whereas the rigid members are themselves jointed, the supplement does not rely on the wearer's joints for sole support.
Devices of this type can be used by persons with weakened muscles or those with nervous system problems which limit the extent to which the person can apply muscular force. Paraplegics, persons having a portion of a limb amputated, aged persons, convalescents and the like can all benefit from power assist devices. In addition, healthy persons can use devices of this type in order to increase their muscle power as needed to perform functions requiring more than normal muscle power or stamina. For example, workmen with power assist devices can lift and carry greater loads, soldiers can carry extra ammunition or body armor, and so forth. These capabilities are achieved without the user operating a separate piece of equipment for lifting, carrying, etc. The wearer's action is intended to be similar to natural muscular and skeletal action, but at greater power.
Where a power assist orthosis is coupled directly to a wearer, the application of force must be limited by the degrees of freedom of the wearer's joints. Prior art attempts to provide power assist devices have been characterized by mechanisms which define idealized and limited rotation axes. The natural degrees of freedom of human joints are not reflected in the joints of the external skeletal members. In Kultsar, for example, the mechanisms associated with the hip joints, knees, elbows, etc. each have only one respective degree of freedom. The mechanism for the hips, for example, defines on each opposite side of the body a rotation axis parallel to the ground surface and passing laterally along the line of the wearer's hip joints. The power applied by the device is thus limited to causing the femur to .swing forward or backward parallel to a vertical plane including the hip joint of the apparatus.
However, the hip joint has additional degrees of freedom, as do other joints of the body. The proximal or hip end of the femur defines a laterally inward bend terminating in a ball which engages the pelvic bone in a ball and socket joint. The hip joint is thus a universal joint capable of positioning the femur at any angle relative to the pelvis and also can rotate the femur relative to its longitudinal axis, subject to maximum limits of angular displacement defined by the ball and socket structures of the joint.
The respective degrees of freedom of a joint can be described with reference to the mutually perpendicular axes of a cartesian coordinate system. A joint such as the hip, shoulder or wrist permits: lateral/medial displacement (for example in the hip joint of a standing person, the rotation of the femur about a substantially vertical axis); flexion/extension (swinging of the leg from front to back about a horizontal axis aligned transversely of the person); and, abduction/adduction (swinging of the leg outwardly and inwardly about a horizontal axis extending forward and rearward). Natural movements such as walking typically include components in each of these degrees of freedom.
If a supplemental joint is modeled to accommodate only certain of the degrees of freedom, it is not possible for the wearer to move naturally. In Kultsar, wherein a hip joint permits only flexion/extension, it is difficult or impossible for the wearer to balance or to move normally using the device. Therefore, even though supplemental power assists in forward/rearward swinging of the leg, the apparatus cannot be used effectively to assist a person in walking or otherwise carrying on normal activities with power assist.
The respective joints vary in their natural limits of displacement as well as in their levels of freedom around the respective axes mentioned. Whereas the hip and shoulder are substantially universal joints, the elbow for example is limited in freedom of abduction/adduction, free in flexion/extension above the aligned position of the forearm and upper arm. Due to the arrangement of the radius and ulna, the forearm between the wrist and elbow is quite free in lateral/medial displacement, allowing rotation of the wrist.
Although it is possible to couple force exerting actuators directly to the user's limbs as described, it is not advantageous in a power assist device to couple to a human joint an external jointed fixture which has the same degrees of freedom as the joint. Whereas the external joint would define degrees of freedom which are centered on the external joint rather than on the biological joint, power applied to the external skeletal members by such a device seeks to move the skeletal members about axes intersecting at the universal joint of the external skeleton rather than axes intersecting at the biological joint. The external device exerts unnatural and potentially injurious force on the bones at the joint because the force is not aligned to axes of freedom of the actual joint between the wearer's skeletal members. An external universal joint of mechanical skeletal members placed alongside the hip joint, for example, defines a center which is displaced laterally from the center of the biological hip joint. As a result, any from the center of the biological hip joint. As a result, any force applied to the external universal joint applies tensile or compressive forces between the two bones of the biological joint. Similar results accrue in other joints. A flexion/extension joint for an elbow, for example, can be placed laterally alongside the elbow, but placing it over or under the elbow applies compression or tension between the bones at the elbow joint, and so forth. In short, it is difficult to accurately reflect the degrees of freedom of a biological joint in an exoskeletal joint, primarily due to the inherent necessity that the exoskeletal joint, which is external to the wearer, be displaced from the biological joint. The problem is acute where a joint has two or more degrees of freedom.